The present invention relates to metal powders for injection molding use, their compounds and the method for producing sintered parts from the same.
(1) Sintered steel, which is a kind of sintered metal body, is partially replacing ingot stainless steel, since the former offers advantages over the latter with respect to improvement of the yield and reduction of machining cost.
With regard to the molding method for the sintered steel, great hopes are entertained of such injection molding methods that will readily enable molding of parts having complex three-dimensional configurations in place of the compression molding whose limitation is that the producible parts are limited to those of two-dimensional designs.
However, since the manufacture of sintered steel bodies by injection molding was started only recently, there are still a variety of technical problems which remain unresolved, and, in particular, there is room for major improvement in the raw material powder.
Generally, it is requisite for the raw material powder for injection molding of a 20 microns or less average particle diameter that it is in the spherical shape and in the form of fine particles. An advantage of the spherical powder is that it imparts good slip among the particles, that is to say, it has excellent injection moldability. For instance, by comparison of a spherical powder with an irregular shape powder, both of which have been added with an organic binder of an identical kind and in an identical quantity, it is found that the former offers a lower viscosity and demonstrates better injection moldability. Furthermore, an equivalent level of injection moldability can be achieved with a lower quantity of the binder. For the said reasons, it becomes possible to shorten the debinding cycle, and also to achieve a high density by dint of a finer particle size of the powder.
For the purpose of achieving such properties required of the raw material powder, modification of operational parameters for the atomizing apparatus, e.g. the pressure and flow rate of the atomizing medium and adjustment of the diameter of the metallic melt injection nozzle, has been the conventionally adopted means. However, no other means for the intended improvement has been adopted through the route of altering the chemical composition of the raw material powder, but such chemical composition that is similar to that adopted for the raw material powder intended for compression molding, whose average particle diameter is about 80 microns, that is to say, the chemical composition form, in which such impurities as may interfere with compressiblity in the case of compression molding have been removed to the bare minimum, has been conventionally adopted.
Nonetheless, problems have been experienced in that satisfactory injection moldability cannot be achieved with fines for injection molding of the conventional chemical composition (Ref.: The "Tokushu-ko (Special Steel)", Vol.36, No.6, page 52, Table 1, Jun. 1, 1987), since spherical particle formation does not take place to a sufficient extent in that powder.
(2) The present circumstance is that being in current use as a raw material powder for the injection molding use are those powders which are essentially sintering fine powders for the compression molding use as described in the Japanese Patent Publication No. 1761/84, the "Funtai Oyobi Funmatsu Yakin (Powders and Powder Metallurgy)", Vol. 12, No. 1 (February, 1965), page 25 to page 32, by Tamura et al. and the "Funtai Oyobi Funmatsu Yakin", Vol. 22, No. 1 (March, 1975), page 1 to page 11, by Kato et al., but have chemical compositions not at all different from those of the powders intended for powder metallurgy, namely, comprising 1.5% or less by weight of silicon, 0.4% or less by weight of manganese, and less than 1 Manganese/Silicon ratio(ordinarily, the Manganese/Silicon ratio is less than 0.3).
What has remained problematic is that the said powders are not necessarily satisfactory with respect to the injection moldability and the sintering characteristics, since the traditional technological philosophy centering on prevention of oxidation in the atomizing step by keeping the Manganese/Silicon ratio at less than 0.3 has been followed strictly so that chemical compositions having extremely reduced contents of carbon and manganese, which deteriorate the compressibility and moldability in the compression molding operation, have been conventionally used, while such practice resulted in insufficient development of such expertise for producing spherical powder and handling oxides on the surface, which is required of stainless steel powder (the average particle diameter: 20 microns or less).
(3) Iron-Cobalt-type alloy is known as a soft magnetic material having the highest saturated magnetic flux density among all magnetic materials. In other words, Iron-Cobalt-type alloy can be said to exhibit a higher magnetic energy with a given volume among all magnetic materials. Great hopes are entertained of this material, by virtue of its excellent magnetic characteristics, for applications associated with electric motors, magnetic yoke, and the like which require high magnetic energy generated from small-size parts.
On the other hand, ingot Iron-Cobalt-type alloy is in such a dilemma that industrial production of small-size parts is virtually impossible due to its poor cold workability.
Powder metallurgy is considered to be a valid means by which to overcome such inferior workability, and variety of methods have been proposed. For instance, there are the Japanese Patent Laid Open No. 291934/86, the Japanese Patent Laid Open No. 54041/87, and the Japanese Patent Laid Open No. 142750/87 concerning Iron-Cobalt-type sintered materials, and the Japanese Patent Publication No. 38663/82 (The Japanese Patent Laid Open No. 85649/80) concerning Iron-Cobalt-type sintered materials containing phosphorus and the japanese Patent Laid Open No. 85650/80 concerning Iron-Cobalt-type sintered materials containing boron. Furthermore, there is the Japanese Patent Laid Open No. 75410/79 concerning Iron-Cobalt-Vanadium-type sintered materials.
However, all of the hitherto proposed methods, which depend on the principle of compression molding, are accompanied by such limitation that so-called mixed powder, namely, a powder prepared by admixing iron-cobalt alloy powder, cobalt-vanadium alloy powder, iron-phosphorus alloy powder, and/or iron-boron alloy powder with iron powder and cobalt powder, so that the raw material powder will enable molding in a mold for the compression molding press, while the admixing or blending ratio had to be limited to an extent that does not deteriorate the compressibility.
For the said reason, it has been the object of the conventional technique to overcome low sintered density and low magnetic characteristics attributable to the said limitations. The method proposed in the Japanese Patent Laid Open No. 291934/86 is intended for improvement in the compressibility by utilizing rapidly quenched iron-cobalt alloy, in which no regular lattice structure is formed, as well sinterability by dint of the blend of such rapidly quenched iron-cobalt alloy powder with cobalt powder, the method proposed by the Japanese Patent Laid Open No. 54041/87 is for improvement in the sintered density by HIP (hot isostatic press) Method, and the method proposed by the Japanese Patent Laid Open No. 142750/87 aims at improvement in the magnetic characteristics by means of improved green density (compressed powder density) and sintered density by combination of coarse Iron-Cobalt-type alloy powder with cobalt fines.
There are methods proposed in the Japanese Patent Publication No. 38663/82 (The Japanese Patent Laid Open No. 85649/80) and the Japanese patent Laid Open No. 85650/80, both of which are intended for improving magnetic characteristics by means of achieving high sintered density that unblended powders. The former method comprises sintering a pulverized iron-phosphorus alloy (26.5% by weight of P) so that the phosphorus content will be 0.05 to 0.7%. The latter method comprises sintering of pulverized iron-boron alloy (19.9% by weight of B) so that the boron content will be 0.1 to 0.4%.
Furthermore, the sintered material disclosed in the Japanese Patent Laid Open No. 75410/79 is intended to improve magnetic characteristics by increasing the sintered density of Iron-Cobalt-Vanadium-type alloy as sintering material through liquid phase sintering of a composition prepared by blending pulverized vanadium-cobalt alloy ground powder consisting of 35 to 45% by weight of vanadium, comprising 38% vanadium eutectic composition, with iron powder and cobalt powder. The conventional methods as proposed hereinabove are, however, intended for compression molding using a mold, and are not applicable to injection molding, since the raw material powder is essentially a mixture of various single-element metal coarse powders having inferior sintering characteristics and two-element alloy powders, and said powders differ from one another in the particle size and the particle shape due to difference in the manufacturing methods employed.
In the present days, Iron-Cobalt-type sintered material is replacing a part of the ingot iron-cobalt alloy material on account of the former's advantage with respect to the yield and the machining cost. In particular, with regard to the molding process, expectation is entertained of future development of the injection molding method which is capable of readily giving three dimensional profile parts, substituting the compression molding method which is merely capable of producing two dimensional parts.
Nevertheless, since it is only recent that the manufacture of Iron-Cobalt-type sintered material, depending on the injection molding technique, was started, there still remain various technical problems which are yet to be resolved. In particular, with regard to raw material powder, there is much to be improved.
Generally, the raw material powder intended for injection molding is required to be fines in the spherical particle shape, and has oxides on the surface of the particle which can be reduced, in the case of Iron-Cobalt-type sintered material as was described in relation to the Stainless Steel-type sintered material.
However, even in the case of Iron-Cobalt-type sintered material, alteration of the chemical composition of the raw material powder has not been adopted as a means for realizing improvement, as was the case with Stainless Steel-type sintered material, but a same composition as the raw material powder (having an average particle diameter of about 80 microns) predicated on an assumption that it be used for the compression molding only has been adopted.
Namely, chemical compositions in which such impurities as will deteriorate the compressibility and the processability in the compression molding step have been conventionally used. However, there existed problems in that said chemical compositions are not necessarily satisfactory with respect to the injection moldability and sintering characteristics, since there are not available sufficient knowledge and experiences regarding the method by which to obtain the spherical particle shape and oxides on the surface for the fine powder for the injection molding use having the conventional composition (the average particle diameter is 20 microns or less).